Abstract. We present a kinetic double-layer surface model (K2-SURF) that describes the
degradation of polycyclic aromatic hydrocarbons (PAHs) on aerosol particles
exposed to ozone, nitrogen dioxide, water vapor, hydroxyl and nitrate
radicals. The model is based on multiple experimental studies of PAH
degradation and on the PRA framework (Pöschl-Rudich-Ammann, 2007) for
aerosol and cloud surface chemistry and gas-particle interactions.

For a wide range of substrates, including solid and liquid organic and
inorganic substances (soot, silica, sodium chloride, octanol/decanol,
organic acids, etc.), the concentration- and time-dependence of the
heterogeneous reaction between PAHs and O3 can be efficiently described
with a Langmuir-Hinshelwood-type mechanism. Depending on the substrate
material, the Langmuir adsorption constants for O3 vary over three
orders of magnitude (Kads,O3 ≈ 10−15–10−13 cm3),
and the second-order rate coefficients for the surface layer
reaction of O3 with different PAH vary over two orders of magnitude
(kSLR,PAH,O3 ≈ 10−18–10−17 cm2 s−1). The
available data indicate that the Langmuir adsorption constants for NO2
are similar to those of O3, while those of H2O are several orders
of magnitude smaller (Kads,H2O ≈ 10−18–10−17 cm3).
The desorption lifetimes and adsorption enthalpies inferred from
the Langmuir adsorption constants suggest chemisorption of NO2 and
O3 and physisorption of H2O. Note, however, that the exact
reaction mechanisms, rate limiting steps and possible intermediates still
remain to be resolved (e.g., surface diffusion and formation of O atoms or
O3− ions at the surface).

The K2-SURF model enables the calculation of ozone uptake coefficients,
γO3, and of PAH concentrations in the quasi-static particle surface
layer. Competitive adsorption and chemical transformation of the surface
(aging) lead to a strong non-linear dependence of γO3 on time and gas
phase composition, with different characteristics under dilute atmospheric
and concentrated laboratory conditions. Under typical ambient conditions,
γO3 of PAH-coated aerosol particles are expected to be in the range of
10−6–10−5.

At ambient temperatures, NO2 alone does not efficiently degrade PAHs,
but it was found to accelerate the degradation of PAHs exposed to O3.
The accelerating effect can be attributed to highly reactive NO3
radicals formed in the gas phase or on the surface. Estimated second-order
rate coefficients for O3-NO2 and PAH-NO3 surface layer
reactions are in the range of 10−17–10−16 cm2 s−1
and 10−15–10−12 cm2 s−1, respectively.

The chemical half-life of PAHs is expected to range from a few minutes on
the surface of soot to multiple hours on organic and inorganic solid
particles and days on liquid particles. On soot, the degradation of
particle-bound PAHs in the atmosphere appears to be dominated by a surface
layer reaction with adsorbed ozone. On other substrates, it is likely
dominated by gas-surface reactions with OH or NO3 radicals
(Eley-Rideal-type mechanism).

To our knowledge, K2-SURF is the first atmospheric process model describing
multiple types of parallel and sequential surface reactions between multiple
gaseous and particle-bound chemical species. It illustrates how the general
equations of the PRA framework can be simplified and adapted for specific
reaction systems, and we suggest that it may serve as a basis for the
development of a general master mechanism of aerosol and cloud surface
chemistry.